The present invention relates to a polypropylene film and more particularly to a polypropylene film having high transparency and heat-sealability and superior in the balance between stiffness (tensile modulus) and impact resistance.
Polypropylenes are superior in mechanical characteristics and also superior in transparency, resistance to heat and chemicals and processability, so polypropylene films have heretofore been used widely in the packaging field.
On the other hand, with the recent diversification of the life mode, the quality of diet have changed and the necessity of a long-period preservation of foods has been increasing to a greater extent. At the same time, the occasion of handling packaged foods at low temperatures has become more frequent. Under the circumstances, polypropylene film having the aforementioned characteristics is one of the most useful materials for packaging use. However, polypropylenes and especially a homopolymer of propylene are weak in impact resistance at low temperatures although they are superior in mechanical characteristics and transparency, and thus they are not satisfactory for the packaging use.
In order to remedy this drawback there have been proposed various methods such as the addition of rubber component to a polypropylene (Japanese Patent Publication No. 7088/1960), the addition of polyethylene thereto (Japanese Patent Publication No. 6975/1962), or the addition of a small amount of ethylene during the polymerization of propylene, allowing a random copolymerization of propylene and ethylene to take place (Japanese Patent Publication No. 11230/1968).
With such proposed methods, however, mechanical characteristics, especially tensile modulus, as well as transparency, are deteriorated although the impact resistance is improved, and so it has been difficult to obtain a fully satisfactory performance.
It is the object of the present invention to remedy the above-mentioned impact resistance without deteriorating mechanical characteristics and transparency.